1. Field of the Invention
The present invention relates to a method and apparatus for vapor deposition of diamond film. More, specifically, it relates to a highly efficient method and apparatus for uniformly vapor depositing a diamond film having superior adhesiveness, superior hardness, and smoothness onto a treated object or substrate and also for depositing a diamond film onto a carburizing material.
2. Description of the Related Art
Diamond is a allotropic form of carbon (C), which exhibits a diamond structure, has a high Mohs hardness of 10, and has a superior thermal conductivity of 1000 or 2000 W/mK, compared with other materials. Therefore, various applications have been developed for using these characteristics.
For example, because of its high hardness, diamond has been considered for use in connection with drill blades or bits. Attempts have been made to cover such tools, which are made of high hardness sintered alloys such as tungsten carbide (WC), with a diamond film. Further, because of its high heat conductivity, diamond has been utilized as a heat sink for LSI, VLSI, laser diode or other semiconductor devices.
When coating a diamond film on a tool made of tungsten carbide (WC) or molybdenum carbide (MoC), even if a chemical vapor deposition apparatus (abbreviated as a "CVD" apparatus) is used similar to that shown in FIG. 1 and the film is grown directly by chemical vapor deposition (abbreviated as "CVD"), the film peels off easily due to differences in the coefficients of heat expansion.
The operation of the above-mentioned known apparatus is as follows.
An object (for example, a tool) to be treated or substrate is placed on a substrate holder 3 cooled by cooling water 2. At the top of the reaction chamber 4 is an anode 6 and a cathode 7 for forming a plasma jet 5. A starting gas 8 is supplied between the anode and the cathode. A DC power source 10 is provided connecting the anode 6 and the cathode 7. At the bottom of the reaction chamber 4 is an exhaust outlet 11. For the CVD growth of diamond, a mixed gas 8 of hydrogen (H.sub.2) and a hydrocarbon, for example methane (CH.sub.4), is supplied so as to flow between the anode 6 and the cathode 7 and into the interior of the reaction chamber 4. The exhaust system is operated to exhaust chamber 4 through the exhaust outlet 11 and the inside of the reaction chamber 4 is held at a low vacuum, in which state are arc discharge 12 is caused between the anode 6 and the cathode 7, the heat of which causes decomposition and plasmatization of the starting gas 8, whereupon the plasma jet 5 including carbon plasma strikes the metal plate 1 and a diamond film 13 composed of fine crystals is grown on the metal plate 1. Thus, it is possible to grow a diamond film 13 on the treated object 1, but since the coefficients of heat expansion differ (for example, the linear expansion coefficient of diamond is 0.0132.times.10.sup.-4 K.sup.-1, while that of W is 0.045.times.10.sup.-4 K.sup.-1) and since the temperature is decreased from the high temperature of 800.degree. C. or more at which the CVD reaction is performed to ordinary temperature, the diamond film 13 easily peels off of the treated object 1.
Therefore, in the prior art, when coating a diamond film on a tool made of WC, for example, elements (for example of Co) included in the WC as sintering reinforcements and for causing a reduction of the adhesiveness were chemically removed, and then the CVD method was used to grow the diamond film. Alternatively, mechanical scratches were made in the substrate and the growth was performed over the same. However, since adhesiveness decreases with film thickness, the thickness of the grown film was limited to several .mu.m, and even so the adhesiveness was insufficient for practical use. The present inventors previously proposed, as a method for resolving this problem, the provision of a coating material layer 15 with a coefficient of heat expansion close to diamond on the treated object 1, as shown in FIG. 2, and the growth of a diamond film 13 on the same (Japanese Unexamined Patent Publication (Kokai) No. 1-145313 published Jun. 7, 1989). However, when actually used, the adhesiveness provided was still not satisfactory for tool use.
As mentioned above, diamond has the highest hardness among all materials, so attempts have been made to use it to form drill blades and bits. However, when diamond is coated on a tool made of WC, for example, the coating easily peels off since the heat expansion coefficients differ and therefore this has not been commercialized.
As mentioned earlier, since diamond has a high heat conductivity, it has been considered for use as a heat sink for semiconductor devices and is being commercialized for this. FIG. 2A is a perspective view of a cooling structure, wherein a heat sink 15 comprised of a diamond is brazed by gold on a subcarrier 14 comprised of cooper (Cu). On top of this heat sink 15 is bonded a semiconductor laser or other semiconductor chip 16 by, for example, gold-tin solder.
FIG. 2B shows the sectional structure of the heat sink 15, wherein a titanium (Ti) film 18, platinum (Pt) film 19, and gold (Au) film 20 are formed in successive layers at thicknesses of about 2000 .ANG. respectively. The reason why the Ti film 18 is used is that it forms titanium carbide (TiC) with the diamond film 17 and has a good adhesiveness. Further, the reason why the Pt film 19 is interposed is so as to correct the poor wettabilities of the Ti film 18 and Au film 20. However, such a heat sink 15 suffers considerably from the effects of the heat conductivities of the metal films enclosing it and from the complicated nature of the structure. Further, the bonding of the separate layers requires high temperature, so the semiconductor chip can easily be damaged, or the bonding requires special skills, so the price becomes high.
With reference to the apparatus of FIG. 1, when plate 1 plated on a substrate holder 3 is cooled by cooling water 2, it becomes possible to form a diamond film 13 thereon.
As mentioned above, diamond has an extremely superior heat conductivity of 2000 W/mK, so it is being commercialized as a component material for heat sinks, but as shown in FIG. 2B, a Ti/Pt/Au metal film is formed in layers on the diamond film. Therefore, there are the problems that the heat conductivity of the diamond is impaired and the cost becomes higher.
Furthermore, as mentioned above, diamond is used for high performance tools utilizing its hardness, and is used as heat sinks for devices where there is a large generation of heat such as laser diodes because it has a high heat conductivity which is several times that of copper. However, the surfaces of conventional thick diamond films have been very rough and the use of these films has required polishing of the surface with a diamond disk etc. This polishing work has required considerable time and labor. The density of nucleus production of diamond is 10.sup.7 to 10.sup.9 cm.sup.-2, a density which is much smaller than the density of other materials (10.sup.13), therefore specific crystal particles selectively grow and the surface becomes very rough. In particular, this trend is striking in the case of thick diamond films (a roughness of about 0.2 mm with film thicknesses of 2 mm).
Furthermore, as mentioned above, attempts have been made to cover tools with diamond to utilize its high hardness. As methods for synthesizing diamonds, there are known the high pressure synthesis method and the low pressure synthesis method. The high pressure synthesis method is suitable for forming relatively large sized monocrystals, but the apparatus is cumbersome and the speed of growth is very slow, so there is the problem of a higher cost. As opposed to this, the low pressure synthesis method includes the microwave plasma chemical vapor deposition method and the electron assisted chemical vapor deposition method. The speed of growth is much higher compared with the high pressure method and it is possible to form diamond as fine crystals on the treated substrate.
Contrary to the above, the present inventors have developed a method for chemical vapor deposition (CVD) of a diamond film using the plasma jet chemical vapor growth apparatus shown in FIG. 1, as mentioned above. However, to cause the growth of the diamond film by plasma jet CVD, a carbon source such as CH.sub.4 or other hydrocarbon must be heated rapidly to a high temperature in the arc discharge region where it is decomposed so as to be ejected as a plasma jet which strikes the metal plate 1 to lose energy whereupon the carbon crystallizes as diamond. Not all of the carbon (C) from the hydrocarbon turns into diamond at this time. Rather, a considerable amount thereof precipitates as amorphous carbon or graphite on the metal plate 1, but when hydrogen gas (H.sub.2) is mixed with the hydrocarbon in the starting gas 8, the amorphous carbon or graphite nondiamond components are reduced to CH.sub.4, C.sub.2 H.sub.6 or other hydrocarbons which are removed as gases.
Actually, however, the removal action of the H.sub.2 gas is not complete and such nondiamond components are detected in most diamonds film formed by the plasma jet CVD method. This has been a factor in reducing the hardness of diamond films.
As mentioned earlier, diamond has a high hardness of 10,000 kg/mm.sup.2, so it has been recognized as a potential coating material for various tools, but the diamond film obtained by the plasma jet CVD method contains amorphous carbon or graphite, resulting in insufficient hardness.
Furthermore, in the past, diamond has been used in connection with high performance tools which utilize its hardness and as heat sinks for devices which generate large amounts of heat such as semiconductor lasers since it has a high heat conductivity of as much as several times that of copper.
However, when a diamond film is formed by CVD, since the material serving as the substrate is a material (a carburizing material such as, for example, Ni, Co, and Fe) through which carbon may permeate, the carbon component has been dispersed in the substrate making synthesis of a diamond film impossible. Therefore, when using a diamond film for a tool or heat sink in the prior art, it has been limited to base materials (an arm in the case of a tool and a subcarrier in the case of a heat sink) other than carburizing materials. However, it would be very convenient if it was possible to form a hard film like diamond on a soft material like a carburizing material.